The present invention relates to coupling of laser diodes into fibers. More specifically, the present invention relates to improving coupling efficiency by improving the spatial brightness of multi-mode diode lasers as they are coupled into double-clad fibers.
Presently, power-scaled fiber lasers are required in a variety of applications, including optical satellite and free space communication links, laser radars, high power fiber delivery systems for manufacturing, and missile guidance systems. Typically, power-scaling in a fiber amplifier is achieved by coupling large-aperture multi-mode laser diodes into the inner cladding of a double-clad fiber. Dopant ions present in the core of the double-clad fiber are optically pumped by the output of the multi-mode diode guided by the inner cladding. The distance required to absorb the pump into the core is proportional to the cross-sectional area of the inner cladding. Thus, the smaller the inner cladding diameter that can guide the pump laser, the more efficiently the pump will be absorbed. Diode brightness is defined as proportional to the product of diode power and etendue, the diffractive solid angle of the output times the diode area. Accordingly, in order to achieve efficient coupling between a single-element diode laser and the fiber, the inner cladding must have a area-solid angle product comparable to or larger than the diode laser output. Unfortunately, the multi-mode diode lasers utilized with double-clad fibers provide high power but relatively highly divergent or low brightness output compared to single mode diodes. The highest quality optics cannot compensate for a lack of brightness in the diode. Note, however, that in multi-element diodes, so-called brightness enhancing optics can be used to eliminate the gaps between the elements, allowing a multi-element diode to maintain the brightness of its individual elements. For example, U.S. Pat. Nos. 5,799,029 and 5,930,280 disclose brightness enhancers for systems wherein multiple single-mode laser sources are used, instead of a multi-mode source, to achieve the necessary power. The outputs of these multiple sources can be essentially folded on top of one another to provide high power output. However, this approach, like other previously reported methods for improving multi-mode diode laser brightness, increase the complexity and size of the laser system.
The present invention provides an apparatus and method whereby the coupling efficiency between a multi-mode diode laser and a double-clad fiber can be enhanced without increasing the complexity or size of the resultant system. In the present invention, the output of a multi-mode diode laser is coupled into both the core and inner cladding of a double-clad fiber. A Bragg grating at the diode laser output wavelength is fabricated into the core of the double-clad fiber. The grating reflects diode laser output from the core back to the diode laser. The reflected feedback enhances the lower order spatial modes at the expense of the higher order modes, and thus enhances the brightness of the resultant diode laser output coupled back into the core and cladding of the fiber. In preferred embodiments of the present invention, a Bragg grating is fabricated in the core of the double-clad fiber by ultraviolet writing techniques known to those skilled in the art. Further, the grating preferably has high absolute reflectivity such that most of the output directed into the core is reflected back to the diode laser. Also, in preferred embodiments of the present invention, the diode laser may be anti-reflection coated to suppress the higher order modes. In addition, in preferred embodiments the bandwidth of the Bragg grating is broadened, for example by spatial chirping, to match the bandwidth of the diode laser such that coupling efficiency is further improved.